1. Field of the Invention
The present invention relates to a position detection apparatus for detecting a position of movement of two members which move relative to each other.
2. Description of the Related Art
Heretofore, a position detection apparatus for detecting a position of movement of two members which move relative to each other is well known. This position detection apparatus comprises a scale on which a periodic signal whose signal level varies in a certain wavelength is recorded along a certain direction, a head section for detecting the periodic signal recorded on the scale, and an arithmetic processing section for performing signal processing of the periodic signal detected by the head section to output position information. The scale and the head section are fitted to a moving element and a base element of two members moving relative to each other. The position detection apparatus detects with the scale the periodic signal whose signal level changes, and supplies the detected periodic signal to the arithmetic processing section. The arithmetic processing section outputs position information showing a position of relative movement of the two members, based on the periodic signal detected by the head section.
The position detection apparatus generally outputs a two-phase increase and decrease pulse, a so-called A/B phase signal, as position information. The two-phase increase and decrease pulse is composed of two signals, A phase signal and B phase signal, which have the same period to each other and whose phases are shifted by xc2xc period. The two-phase increase and decrease pulse is a signal showing a Gray coded two-bit count value, with the A phase signal being designated as a lower bit, and the B phase signal being designated as an upper bit. This count value shows a quantity of relative movement. In this manner, if the two-phase increase and decrease pulse is transmitted as position information, the quantity of information to be transmitted can be reduced. Moreover, since the displacement is shown using the Gray code, the direction of movement can also be transmitted clearly. Then, a control unit or the like which obtains the two-phase increase and decrease pulse from the position detection apparatus 1 as position information cumulatively adds (subtracts when two members move in the negative direction) the number of count of the two-phase increase and decrease pulse, to thereby determine the quantity of relative movement of two members, thereby controls a position of movement of two members.
With a conventional position detection apparatus, there is a case where the count speed of the two-phase increase and decrease pulse to be output may abruptly increase due to noise, quantize error, impact, vibration or the like, and may exceed the allowable speed temporarily. In such a case, the two-phase increase and decrease pulse is not counted for each count, and, for example, there is a possibility that a pulse is omitted for more than one count and the order of the count value is last. Thus, if the order of the count value is last, the control unit which has obtained the two-phase increase and decrease pulse cannot cumulatively add the count number, and cannot detect the displacement of a machine tool.
Moreover, with a conventional position detection apparatus, there is a case where though the two members to be measured are stationary, the count output of the two-phase increase and decrease pulse is not stable due to noise, quantize error or the like. In such a case, the output two-phase increase and decrease pulse fluctuate to the positive and negative sides, and the control unit is informed that the two members are in a slightly moving condition, though stationary.
It is an object of the present invention to provide a position detection apparatus capable of producing a stable output signal.
The position detection apparatus according to the present invention comprises: a recording medium on which a position signal comprising a periodic signal is recorded; a detection section comprising a first detection head which moves relative to the recording medium along the recording direction of the position signal for detecting the position signal, and a second detection head which is disposed apart from the first detection head by a predetermined distance in the recording direction of the position signal, and moves relative to the recording medium, operating together with the first detection head for detecting the position signal; a polar conversion section for converting the position signal detected by the first detection head and the second detection head into an angle signal showing a relative position of the recording medium and the detection section in one period as an angle; a response limiting section for limiting response of the angle signal; and an output section for outputting relative position information of the recording medium and the detection section, based on the angle information in which response is limited by the response limiting section, characterized in that the response limiting section limits response such that when an absolute value of a variation in the angle signal exceeds a predetermined threshold value, the absolute value of the variation in the angle signal is the predetermined threshold value.
With the position detection apparatus, response is limited by means of a through rate limitation such that when the absolute value of the variation in the angle signal exceeds a predetermined threshold value, the absolute value of the variation in the angle signal is the predetermined threshold value.
Moreover, with the position detection apparatus according to the present invention, the response limiting section limits response such that when the absolute value of the variation in the angle signal is a first threshold value or smaller, the angle signal does not change, when the absolute value of the variation in the angle signal is or exceeds a second threshold value which is larger than the first threshold value, the absolute value of the variation in the angle signal is a value obtained by subtracting the first threshold value from the second threshold value, and when the absolute value of the variation in the angle signal is larger than the first threshold value and smaller than the second threshold value, the absolute value of the variation in the angle signal is a value obtained by subtracting the first threshold value from the absolute value of the variation.
With this position detection apparatus, the response limiting section limits the response such that when the absolute value of the variation in the angle signal is the first threshold value or smaller, a hysteresis range is provided so that the angle signal does not change, and when the absolute value of the variation in the angle signal is or exceeds the second threshold value, the response limitation by means of a through rate limitation is performed so that the absolute value of the variation in the angle signal is a value obtained by subtracting the first threshold value from the second threshold value, and when the absolute value of the variation in the angle signal is larger than the first threshold value and smaller than the second threshold value, the response limitation is performed so as to follow linearly, delayed by the hysteresis, so that the absolute value of the variation in the angle signal is a value obtained by subtracting the first threshold value from the absolute value of the variation.
With the position detection apparatus according to the present invention, when the absolute value of the variation in the angle signal is or exceeds a predetermined threshold value, the position detection apparatus limits the response by means of a through rate limitation so that the absolute value of the variation in the angle signal is the predetermined threshold value.
As a result, with the position detection apparatus, since the through rate limitation is carried out, the change speed of the output signal does not exceed the allowable speed, and a stable output signal can be obtained.
Moreover, according to the position detection apparatus of the present invention, the response limiting section limits the response such that when the absolute value of the variation in the angle signal is the first threshold value or smaller, a hysteresis range is provided so that the angle signal does not change, and when the absolute value of the variation in the angle signal is or exceeds the second threshold value, the response limitation by means of a through rate limitation is performed so that the absolute value of the variation in the angle signal is a value obtained by subtracting the first threshold value from the second threshold value, and when the absolute value of the variation in the angle signal is larger than the first threshold value and smaller than the second threshold value, the response limitation is performed so as to follow linearly, delayed by the hysteresis, so that the absolute value of the variation in the angle signal is a value obtained by subtracting the first threshold value from the absolute value of the variation.
As a result, with this position detection apparatus, since a hysteresis range is provided so that the angle signal does not change, the output does not vary when there is no movement such as in a stationary condition or in a condition close to the stationary condition, and hence a stable output can be performed. In addition, with this position detection apparatus, since the through rate limitation is carried out, the change speed of the output signal does not exceed the allowable speed, and a stable output signal can be obtained.